1. Field of the Invention
The present invention relates to a crystal diameter measuring device for measuring the diameter of the growing portions of single crystals grown by the Czochralski (CZ) technique.
2. Description of the Related Art
The Czochralski single crystal growth apparatus is designed to grow a single crystal 20 by immersing a seed crystal 18 held at the lower end of a wire 14 with a holder 16 into a melt generated by heating a polycrystalline material accommodated in a quartz crucible 10 by a heater (not shown) provided around the quartz crucible 10 and then by pulling the seed crystal 18 up with wire 14 from the melt 12, as shown in FIG. 11. The single crystal 20 has a neck portion 22 whose diameter is narrowed from about 10 mm of the diameter of the seed crystal 18 to 2 to 5 mm in order to push out dislocations from the bulk, a conical portion 24 whose diameter increases gradually, and a cylindrical body portion 26 having a diameter of, for example, 150 mm and used for preparing wafers.
To make the shape of the thus-prepared single crystal 20 coincide with a desired pattern, the diameter D of the growing portion of the single crystal 20 is measured. This is accomplished by detecting the diameter of the image of a bright ring 27 formed on an interface between the single crystal 20 and the melt 12, because the diameter D of the growing portion is in proportion to the diameter of the image of the bright ring 27.
To measure both the diameter of the relatively thin neck portion 22 and that of the relatively thick cylindrical portion 26 with a high degree of accuracy, a measuring method has been proposed (in Japanese Patent Laid-Open No. 87482/1987) in which the diameter of the growing portion of the crystal is measured by obtaining an image thereof using a two-dimensional camera with a zoom lens and then by processing the obtained image.
However, accurate measurements of the diameter spreading over a range from 2 mm to 150 mm require a zoom lens having a magnification of at least 6. Furthermore, a zoom lens having a large magnification has a narrow field of view. Consequently, when the vertical position of the surface of the melt 12S changes, the position of the camera must be moved in the vertical direction accordingly, or the angle of inclination of the optical axis of the camera must be changed accordingly.
Where
L: subject distance (mm) PA0 X: lateral field of view (mm) PA0 f: focal distance of a lens (mm) PA0 D: lateral resolution of a camera (mm/bit),
we have the following equations: EQU L=(1+1/8.8 X)f (1) EQU D=X/756 (2)
In the case of the measurements of the diameter of the neck portion 22, f=180, L=900, X=35.2, and D=0.05.
In the case of the measurement of the diameter of the cylindrical portion 26, f=30, L=900, X=255.2, and D=0.33 (which is not good).
To overcome the aforementioned problems, the method of measuring the diameter of the single crystal 20 using a one-dimensional camera 28, as shown in FIG. 11, has been used (Japanese Patent Laid-Open No. 112493/1988).
In this method, a line sensor 30 of the one-dimensional camera 28 is desirably disposed in the plane containing the wire 14, and the image of the point where the plane intersects the bright ring 27 is formed on the line sensor 30 by a lens 32. The diameter D of the single crystal 20 is detected from the pixel position X of the bright point. The diameter D is also dependent on both the horizontal distance R from the central line of the single crystal 20 to the central point of the line sensor 30 and the vertical distance H from the melt surface 12S to the central point of the line sensor 30. Since the distance R is constant, the diameter D is determined by X and H. This is expressed by D=F (X, H). The diameter D is dependent on the height H also in the measurements which use the two-dimensional camera.
A camera with an area sensor is generally used as the two-dimensional camera. The number of pixels of one line of such a camera is about 512 pixels at maximum, whereas the number of pixels of a line sensor 30 is, for example, 2048 or 4096. In the measuring method which uses the one-dimensional camera, since the point where the plane containing the wire crosses the bright ring 27 is detected, it is possible to accurately measure the diameter D of the single crystal 20 by the fixed one-dimensional camera 28 even when the height of the melt surface 12S is varied.
However, measuring the diameter of the neck portion 22 using this one-dimensional camera 28 has a drawback in that, since the diameter D is relatively small, vibrations of the neck portion 22 in the horizontal direction may preclude the image of the bright point from being focused on the line sensor 30. This makes measurements of the diameter D impossible.
Furthermore, the present inventors have discovered that measuring the diameter D of the neck portion 22 by the one-dimensional camera 28 involves the following problem.
FIG. 12 shows the opening and closing states of a shoulder chamber 34 which covers a main chamber from above. An arm 36 protrudes from the peripheral surface of the shoulder chamber 34. When the polycrystal silicon is accommodated in the quartz crucible 10, the shoulder chamber 34 is rotated about a shaft 38 clockwise as viewed in FIG. 12 by 90 degrees to open the upper opening of the main chamber. After a desired polycrystal has been accommodated within the quartz crucible 10, the shoulder chamber 34 is rotated in the reverse direction by 90 degrees to close the upper opening of the main chamber. Since the distance between the center of the shaft 38 and the center of the shoulder chamber 34 is as large as, for example, 700 mm, a slight shift in the rotational angle of the shoulder chamber 34, which would be generated by the opening/closing operation, causes a shift `d` in the position of the shoulder chamber 34 to be generated. When this shift `d` in the position was measured, the maximum value of 1.5 mm was observed.
In the method which uses the one-dimensional camera, this shift `d` in the position influences the precision of the diameter measurement.